利用USART1串口输入大量字符，然后用DMA方式将这些字符信息传输到主存。统计这些字符中字母字符的个数，并由UASRT1串口显示统计结果。代码

以下是基于STM32F4的代码实现：

#include "stm32f4xx.h"
#include "stdio.h"

#define BUFFER_SIZE 256

char buffer[BUFFER_SIZE];
int letter_count = 0;

void USART1_Init(void)
{
  GPIO_InitTypeDef GPIO_InitStruct;
  USART_InitTypeDef USART_InitStruct;
  DMA_InitTypeDef DMA_InitStruct;
  
  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);
  RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1, ENABLE);
  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_DMA2, ENABLE);
  
  GPIO_InitStruct.GPIO_Pin = GPIO_Pin_9 | GPIO_Pin_10;
  GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF;
  GPIO_InitStruct.GPIO_OType = GPIO_OType_PP;
  GPIO_InitStruct.GPIO_PuPd = GPIO_PuPd_UP;
  GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;
  GPIO_Init(GPIOA, &GPIO_InitStruct);
  
  GPIO_PinAFConfig(GPIOA, GPIO_PinSource9, GPIO_AF_USART1);
  GPIO_PinAFConfig(GPIOA, GPIO_PinSource10, GPIO_AF_USART1);
  
  USART_InitStruct.USART_BaudRate = 115200;
  USART_InitStruct.USART_WordLength = USART_WordLength_8b;
  USART_InitStruct.USART_StopBits = USART_StopBits_1;
  USART_InitStruct.USART_Parity = USART_Parity_No;
  USART_InitStruct.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
  USART_InitStruct.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
  USART_Init(USART1, &USART_InitStruct);
  
  DMA_InitStruct.DMA_Channel = DMA_Channel_4;
  DMA_InitStruct.DMA_PeripheralBaseAddr = (uint32_t)&USART1->DR;
  DMA_InitStruct.DMA_Memory0BaseAddr = (uint32_t)buffer;
  DMA_InitStruct.DMA_DIR = DMA_DIR_PeripheralToMemory;
  DMA_InitStruct.DMA_BufferSize = BUFFER_SIZE;
  DMA_InitStruct.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
  DMA_InitStruct.DMA_MemoryInc = DMA_MemoryInc_Enable;
  DMA_InitStruct.DMA_PeripheralDataSize = DMA_PeripheralDataSize_Byte;
  DMA_InitStruct.DMA_MemoryDataSize = DMA_MemoryDataSize_Byte;
  DMA_InitStruct.DMA_Mode = DMA_Mode_Normal;
  DMA_InitStruct.DMA_Priority = DMA_Priority_VeryHigh;
  DMA_InitStruct.DMA_FIFOMode = DMA_FIFOMode_Disable;
  DMA_InitStruct.DMA_FIFOThreshold = DMA_FIFOThreshold_Full;
  DMA_InitStruct.DMA_MemoryBurst = DMA_MemoryBurst_Single;
  DMA_InitStruct.DMA_PeripheralBurst = DMA_PeripheralBurst_Single;
  DMA_Init(DMA2_Stream2, &DMA_InitStruct);
  
  USART_Cmd(USART1, ENABLE);
  DMA_Cmd(DMA2_Stream2, ENABLE);
}

int main(void)
{
  USART1_Init();
  
  while (1)
  {
    if (DMA_GetFlagStatus(DMA2_Stream2, DMA_FLAG_TCIF2) != RESET)
    {
      DMA_ClearFlag(DMA2_Stream2, DMA_FLAG_TCIF2);
      for (int i = 0; i < BUFFER_SIZE; i++)
      {
        if ((buffer[i] >= 'a' && buffer[i] <= 'z') || (buffer[i] >= 'A' && buffer[i] <= 'Z'))
        {
          letter_count++;
        }
      }
      printf("Letter count: %d\r\n", letter_count);
      letter_count = 0;
    }
  }
}